Sandia builds first diamond micromachine

ALBUQUERQUE, N.M.  The first micromachine made of diamond is being designed and prototyped at Sandia National Laboratories. The full design consists of a tiny comb drive the lab created that is linked to a diamond piston and powered by alternating current. The part was etched using 2-micron design rules from amorphous diamond, the second-hardest material after the crystalline diamond used for jewelry.

Diamond has long interested nanotechnology researchers not only because it is durable but also because it is biocompatible. Diamond is just another form of carbon, which is chemically benign inside the human body. Crystalline diamond, which theoretically should perform even better than amorphous diamond, requires impracticably high temperatures to synthesize. Amorphous diamond was also at one time impractical, but Sandia researchers John Sullivan and Tom Friedmann overcame the problems with the material.

Pressure relieved

"We have patented a thermo-annealing method for amorphous diamond that relieves the hundreds of atmospheres of pressure that previously made it impractical. Our annealing process converts some of these stressful diamond bonds into graphite, relieving the internal stresses," Sullivan said.

Polysilicon, the usual material from which micromachines are built, is much softer than diamond. Polysilicon microelectromechanical systems have already been successfully deployed in automobile air-bag sensors and for optical micromirrors. Equivalent diamond micromachines, however, should be several orders of magnitude longer-lived than polysilicon micromachines. "Some experts estimate that diamond could last 10,000 times longer than polysilicon in applications where wear is an issue," Friedmann said.

Besides being durable and biocompatible, diamond also overcomes the "stiction" problem that plagues alternative materials. Stiction describes the force required to make two stationary materials slide against each other. For example, put any two materials together  say, a book lying on a table  and a stiction force is required to make the book slide across the tabletop.

Once the book is in motion, however, the stiction disappears and a much smaller force is required to keep it in motion.

For micromachines, the stiction force required to get stationary parts moving can be even more disproportionate than the force required to keep them in motion. For macroscopic objects like books and tabletops, the stiction force is of the same order of magnitude as the force required to keep it in motion, but for micromachines the stiction force can sometimes be orders of magnitude larger.

"Other researchers are trying all kinds of methods to coat their micromachines with non-stick surfaces, but we don't have to because diamond doesn't like water," Sullivan said. The major contributor to stiction, he said, is water molecules from humidity in the air. The water molecules ordinarily "glue" together the micromachine parts until they are overcome by a force exceeding the force of attraction. However, water molecules naturally avoid bonding to the surface of diamond, so stiction is not a problem for diamond.

Using the patented fabrication method for this first device, it takes about three hours to deposit the diamond using pulsed-laser deposition. The thermo-annealing step that is done to reduce the internal stresses within the material takes only a few more minutes.

The first device fabricated with the new method was a diamond comb driver for an ac-driven piston in a microengine. The comb has tiny teeth spaced just 2 microns apart that move back and forth each time the current reverses direction. The researchers are now building successive layers of amorphous diamond on top of the comb layer to create the piston and gears for the rest of the engine.

The final engine will measure a millimeter square and will consist of two diamond combs lying on a flat surface with their teeth facing each other. One comb is fixed in place and the other is attached to a spring. A diamond piston, fixed to the opposite side of the movable comb, then moves back and forth as the teeth of the combs alternately attract and repel each other. The moving diamond piston will then drive a gear.

A paper describing the diamond deposition method will be presented at the American Vacuum Society's International Conference on Metallurgical Coatings and Thin Films in April in San Diego.